1. Field of the Invention
The present invention relates to an electrically connecting structure for connecting one terminal to another terminal with an anisotropic conductive adhesive, which terminals are arranged to face each other.
2. Description of the Prior Art (Related Art)
Generally, a liquid crystal display device comprises a liquid crystal display panel, semiconductor chips (electronic parts) for driving the liquid crystal display panel, and a circuit board for controlling the semiconductor chips.
FIG. 4 is a view showing an example of the conventional liquid crystal display device described above. This liquid crystal display device comprises a liquid crystal display panel 1, two semiconductor chips 2 and 2a, a flexible wiring board 3, and a circuit board (not shown) which controls the semiconductor chips 2 and 2a. The liquid crystal display panel includes an upper transparent board 4 and a lower transparent board 5, which are rectangular and arranged to face each other, and a liquid crystal material (not shown) which is interposed between the upper transparent board 4 and the lower transparent board 5. The size of the lower transparent board 5 is larger than that of the upper transparent board 4. When the upper transparent board 4 is mounted on the lower transparent board 5 in an overlapping manner with the two adjacent sides thereof meeting with the corresponding adjacent sides of the lower transparent board 5, then a portion of the lower transparent board 5 that is not covered with the upper transparent board 4 can be seen when viewed from the top, as shown in FIG. 4. On the portion (an extended portion) of the lower transparent board 5 that is not covered by the upper transparent board 4, output connecting terminals (output wirings) 6, input connecting terminals (input wirings) 7, and power connecting terminals (power wirings) 8 for a power source are formed. One semiconductor chip 2 is mounted on the extended portion of the lower transparent board 5. In this case, as shown in FIG. 5, input connecting bumps 11, power connecting bumps 12 for a power source, and output connecting bumps (not shown) are formed on a lower surface of the semiconductor chip 2. The input connecting bumps 11 formed on the lower surface of the semiconductor chip 2 are respectively electrically connected to the input connecting terminals 7 formed on the lower transparent board 5. One group of the power connecting bumps 12 formed on the lower surface of the semiconductor chip 2 are electrically connected to one of the power connecting terminals 8 formed on the lower transparent board 5, and another group of the power connecting bumps 12 (12a) are electrically connected to another of the power connecting terminals 8 (8a). The output connecting bumps formed on the lower surface of the semiconductor chip 2 are respectively electrically connected to the output connecting terminals 6 formed on the lower transparent board 5.
Another semiconductor chip 2a is also mounted on the extended portion of the lower transparent board 5 in the same way. A flexible wiring board 3 is linked at one side thereof to the extended portion of the lower transparent board 5, so that wirings (not shown) which are formed on the flexible wiring board 3 are electrically connected to the input connecting terminals 7 and the power connecting terminals 8 of the lower transparent board 5, and another side of the flexible wiring board 3 is linked to the circuit board which controls the semiconductor chips 2 and 2a.
FIG. 6 is a cross sectional view showing an example of an electrically connecting structure for connecting the connecting terminals 7 and 8 of the lower transparent board 5 to the connecting bumps 11 and 12 of the semiconductor chip 2. In FIG. 6, the input connecting terminals 7 of the lower transparent board 5 are electrically connected to the input connecting bumps 11 of the semiconductor chip 2 through an anisotropic conductive adhesive 13. The anisotropic conductive adhesive 13 is made of an insulating adhesive 16 which contains a large number of conductive particles 15, and is formed into a tape-shaped member. The conductive particles 15 are covered with an insulating layer 14 and mixed with the insulating adhesive 16 at an appropriate ratio. At first, the anisotropic conductive adhesive 13 is arranged between a portion of the lower transparent board 5, on which the input connecting terminals 7 are formed, and a portion of the semiconductor chip 2, on which the input connecting bumps 11 are formed, so that a lower surface of the insulating adhesive 16 faces the input connecting terminals 7 and the lower transparent board 5, and an upper surface of the insulating adhesive 16 faces the input connecting bumps 11 and the semiconductor chip 2.
Then the portion of the lower transparent board 5, on which the input connecting terminals 7 are formed, and the portion of the semiconductor chip 2, on which the input connecting bumps 11 are formed, are bonded together with the anisotropic conductive adhesive 13 under heat and pressure. During the bonding process, portions of the insulating adhesive 16, which are put between the input connecting terminals 7 and the conductive particles 15 and between the input connecting bumps 11 and the conductive particles 15 escape therefrom under pressure applied to the lower transparent board 5 and the semiconductor chip 2 to bond them together in the beginning. Thereafter, portions of the insulating layers 14 on the conductive particles 15, which portions come into contact with the input connecting terminals 7 and the input connecting bumps 11, are consequently broken. Hence the conductive particles 15 are brought into contact with the input connecting terminals 7 and the input connecting bumps 11. At this time, other portions of the insulating layers 14 are not broken and still remain on the conductive particles 15. Hence the conductive particles 15 are brought into contact with the input connecting terminals 7 and the input connecting bumps 11. As a result, the input connecting terminals 7 and the input connecting bumps 11 are electrically connected to one another through the conductive particles 15 of the anisotropic conductive adhesive 13. Thereafter, the insulating adhesive 16 and the insulating layer 14 are hardened so that the portion of the lower transparent board 5, on which the input connecting terminals 7 are formed, adheres firmly to the portion of the semiconductor chip 2, on which the input connecting terminal 11 are formed. In this case, if the conductive particles 15 are linked to each other, insulation between the conductive particles 15 is sufficiently kept by insulating layers 14 which are still left on the conductive particles 15.
However, in such a conventional anisotropic conductive adhesive 16, when the anisotropic conductive adhesive 16 is heated and pressured so that the portions of the insulating layers 14, which portions come into contact with the input connecting terminals 7 and the input connecting bumps 11, are broken, thin films of the insulating layers 14 are still left on the portions of the surfaces of the conductive particles 15, which portions face to the input connecting terminals 7 and the input connecting bumps 11, according to circumstances. Therefore, when a pitch of the connecting bumps 11 and 12 of the semiconductor chip 2 is about 60 .mu.m, it is difficult to achieve an electric current flow of about 10 mA from the input connecting terminal 7 of the lower transparent board 5 into the input connecting bump 11 of the semiconductor chip 2 through the conductive particles 15 of the anisotropic conductive adhesive 13. Further, it is needed to achieve an electric current flow of about 200-300 mA from the power connecting terminal 8a of the lower transparent board 5 into the power connecting bumps 12a of the semiconductor chip 2 under some conditions for driving the liquid crystal display panel 1. In such a case of connecting the power connecting terminal 8a to the power connecting bumps 12a, it is needed to provide at least 20-30 power connecting bumps 12a on the semiconductor chip 2. Hence the semiconductor chip 2 is getting larger and high-priced.